soft soil foundation options

[darkalley]

I own a small cottage in the albany ny area.  the building is a 20 ft x 28ft cottage built about 40 years ago. 1 floor + a loft. the building sits on blocks on each corner and in the middle.  they have sunk over time.  the land it sits on is water front that had various fill put down 50 years ago to make it level.  If I dig down past 25 inches the hole begins to fill up with water.  Also, the soil below the top soils is a black sand.  Part of the building now sits on the ground.  I would like to lift it up about 2 feet but what do I use for footings?  This is a summer place.  The sandy soil makes me a bit nervous.  Any ideas??  OK after some research my novice idea is every eight feet 28 inch bigfoot footings.  thoughts??

[Amanda_931]

Might have an idea, even two

But it's definitely pay out the bucks to check with professionals in the area time.

I ran into this while looking for something quite different last week.  Since one thing I'm considering building is on fill dirt, I thought maybe I'd need it one of these years.

http://www.abchance.com/ho_app_newfoundations.html

They talk about drilling and fastening lots of augers down deep enough--sometimes over 25-35 feet to hit solid soil--below your quicksand, if that's what that is.

I gathered from buying plans for an Alaska Log Cabin" once (under 10 dollars, definitely not real blueprints) that in Alaska they sometimes put an monolithicl slab/shallow footing on the permafrost, on the grounds that it spreads out the load enough to float the building, even if it is a log cabin.

[John Raabe]

Amanda:

That auger pier system is interesting and a good suggestion. It might be a reasonably cost effective solution for darkalley's soil problems.

There are lots of places where houses should not be built. Of course, they are built there anyway and then overcoming those problems becomes an ongoing issue.

Frank L. Wright did a floating slab for a big hotel in Tokyo in the early part of the 20th century - it was on fill that would liquify in an earthquake. A workable if expensive solution.

Indegineous folks building in swamplands build up on stilts and then use flexible materials that can be adjusted. Perhaps a modern version is some sort of stilt-pier system with adjustable screw jacks at the top that can be releveled every few years when the marbles start to run to one corner...

[glenn kangiser]

I have worked with the anchors Amanda mentioned in a building remodel. They are screwed into the ground with a Bobcat and a special driver that sets them at a pre-determined torque.  They worked on that job although the contractor put them in the wrong place.

My dads house was built over a wet area at the confluence of two creeks.  Its been sinking for 60 years or more.  Every couple of years they jack it up and put more posts and support.  I think my brother about has a handle on it now.  More support on wider area helps to float the weight.  Maybe pads of treated lumber like they use under mobile homes would spread the weight out at a reasonable price.

As Amanda said, an engineer could prove useful but his solutions could be costly.

Reading a little more of your post - I don't know if I would try to go into the ground and lose what support I had - doesn't make sense to try to put footings into permanent mud unless you went to expensive pilings as mentioned above.  The float idea might work better.  Also jack fairly evenly in small increments as things tend to snap, crackle and pop as they go up.  Again -these are just ideas-- those and $4.50 will get you a cup of coffee at Starbucks.  Just suggestions - you decide.

[Jim]

Thanks for the info.  I have thought about the helictical(sp) piers down the road when a build a more perm structure.  My plan is to take my little cottage and make it usable for the next 5-10 years.  It consists of one bedroom, a small kitchen, real small bath living room(again small) and a porch.  For this structure I'm trying to save as much from a cost standpoint as possible.  Maybe I'm dreaming that the bigfoot piers will work?

[John Raabe]

For a shorter term solution I would do what Glenn suggests - a pad of PT lumber (perhaps 2 layers of double 2x12's - would give you a 23" square) and jack and shim to level. Repeat as needed.

[DavidLeBlanc]

F. L. Wright's Imperial Hotel design literally floated at all times: it was built on a shallow, water filled pan to provide isolation from the subsoil of Japan's Kanto Plain, which is largely glacial fill/sand.

The pan sat on the ground until the soil liquified at earthquake time, at which time it floated on the liquified soil.

Built just a year before the great Tokyo earthquke of 1901 (1902?) it survived largely unscathed, with only cosmetic damage. Totally validated Mr. Wright's innovation.

The hotel was finally pulled down in the 60's or 70's I believe.

[Jim]

OK.  Can I get a little more of an explanation on the float approach??  I think that Idea might be a good temp solution.  So my building is a square.  where am I putting the lumber??  In what direction?  I'm going to look online for some pics of this approach.  Thanks for the advice thus far...

[JRR]

Darkalley
I would suggest you think of "floating bed" footings made of new railroad cross-ties.  Cross-ties laying, on their side, atop the ground will give you a great deal of floatation.   Three or four at each building corner may be enough.  If you are in a hurricane prone area you will need to install anchors accordingly.

[Jim]

my well is near my house so do I have to be worried about the coating on railroad ties seeping into the ground water??  Do they still have Creosote on them??

[glenn kangiser]

JRR - I was thinking about this today and was thinking the same thing.  RR ties could be placed on the ground under the main floor beams then jack and block between them as required.  This may require temporary beams and jacking next to the location of the tie that is to set on the ground to get clearance to install it or doing the worm under the cabin with a shovel to get space dug out.  Not fun but I've done it. :-/

As John said - the PT lumber pads will work also if clearance is a problem.  The general idea is that you have to jack up the main members to support the floor joists and the weight of the house.  If your soil will only support a few hundred pounds per square foot you need a lot more support surface on the ground than you would if it would support 2000 lbs per square foot.  Your jack will tell you a lot when you start jacking the house up -- if the jack goes down and the house doesn't go up then you need a larger pad under the jack and similar for the house.  That is why a RR tie would be good on the ground-- a lot of bearing surface.  It's like you are trying to make a log raft of RR ties to float your house on the wet sand.  Wet sand can carry around 30% water or somewhere in there as I remember from my well drilling days.  No support there.  

[Jim]

Guys I really like this idea.  I think it is an inexpensive solution that fits my needs.  true there will be some future jacking but the interior is paneling/ drop ceiling so I'm not to worried about cracking.   Thanks for all the great advice.  I do know they make 8x8 pressure treated lumber at the very least.  

[glenn kangiser]

I assume with all the water there that you have a shallow well so, yes - contamination could be a problem -with creosote and CCA treated lumber.   You could use concrete pads - a bit alkaline but not much problem.  You could even use bags of concrete mix with gravel in it- wet it - jam a couple rebars in and shove it under the house paper and all-  some moisture and it will get hard right in the bag or use pre cast concrete pads..  If that is high enough to stay dry so you won't get runoff for contamination you could go up with treated lumber from there.  You could also pour concrete pads but that gets harder.

How far and how deep is your well?  Do you know if it has a proper seal?  What type -perforated wells are more subject to contamination than other types though most are perfed or screened.  In the valley here open bottom wells are common.  They are designed to seal contamination out of the lower aquifers.  The above are just things to think about.

[JRR]

I have seen some modern RR crossties made of steel reinforced concrete.   They are usually wide at the bottom, narrow at the top.  Would that be a "trapezoidal"(sp?) cross-section?  You could pour your own ... perhaps in-place... or, at least close-by.   All the rebar should be close ( within 2") of the bottom of the cross-section.

Take a tip from the railways... they nearly always use a ballast bed of gravel under the ties.  This is extra important under wooden ties.

[Jim]

Not 100% sure what you mean when you say the rebar shoud be within 2 inches of the cross section.  Could you elaborate?  I'm going to investigate the concrete option as well and compare costs.  If I go with concrete what size rebar should I use and what PSI concrete? If I make concret pads at each corner how thick and wide should they be??

[Amanda_931]

 _____________
|_____________|

If this is the "section" looking onto that trapezoidal cross-tie from down the track--since the top and bottom are parallel you don't see the slants, then


  _________
 /        ..        \
/__________\

might accidentally be the cross section, and the two little periods the rebar.  Except that they should be within two inches of the bottom.

I love ASCII art, but I'm not much good at it.

[JRR]

Amanda has made a pretty good sketch of the cross-tie.

For ground contact applications, the rebar is to be to have a concrete "cover" to prevent accelerated rusting and to assure good rebar-concrete bonding.   2" probably would not be enough to satisfy any building code, but would probably meet the actual need of a member in compression ....such as a footing that can be watched.  If you are in a coded area, please find what is necessary per code.

How large a total footing area, how thick, what depth, how many rebars of what size,  placed how?  These are questions dependent on local soil conditions, the weight of the structure, and local code requirements.  

I always am very conservative with footing design....don't want cracks.